The present invention relates to an ink-jet printing apparatus and method for printing a desired visible image by forming ink dots on a print medium by discharged ink droplets, and a computer readable memory.
Conventionally, as information processing apparatuses, such as a copying machine, wordprocessor, computer, and the like, and communication apparatuses have prevailed and advanced, an apparatus for printing a digital image using an ink-jet print head has increasingly become prevalent as one of image forming (printing) apparatuses for those apparatuses. Also, since the aforementioned information processing apparatuses and communication apparatuses adopt high-quality, color graphic information, a printing apparatus is required to output a high-quality, color image.
Such printing apparatus uses a print head which comprises a print element array prepared by integrating and lining up a plurality of print elements, and on which a plurality of ink ejection ports and ink channels are integrated at high density, so as to miniaturize pixels. Furthermore, to obtain color outputs, the printing apparatus normally comprises a plurality of print heads corresponding to, e.g., cyan, magenta, yellow, and black inks.
However, high-density integration of the ink ejection ports and ink channels is limited and, hence, miniaturization of pixels has a given limit. In such case, since dots that form each pixel become relatively large, they give rise to graininess in a low-density image highlight portion or the like, and disturb improvement of the image quality of an image.
By contrast, a so-called multi-drop scheme is known. In this scheme, in place of increasing the integration density of ink ejection ports and ink channels (i.e., in place of reducing one pixel size), the dot size of ink to be discharged is reduced and one pixel is formed by ink droplets corresponding in number to a print density. With the multi-drop scheme, since the ink dot size printed on a print sheet can become relatively small, graininess in a low-density portion such as a highlight portion or the like can be eliminated.
However, miniaturization of the ink droplet size is limited in consideration of ejection stability upon discharging small ink droplets, and bottlenecks the improvement of image quality accordingly. In this scheme, since the number of ink droplets to be discharged per pixel increases, the print speed lowers, and conflict between high image quality and high print speed occurs.
As another scheme for improving image quality without increasing the integration density of ink ejection ports, a multi-density printing scheme using multi-density (dark and light) inks of the same hue having different ink densities is known. In this scheme, a highlight portion is printed using low-density light ink to reduce graininess of ink dots, and a high-density portion is printed using dark ink. Since this scheme need not increase the number of ink droplets to be discharged upon forming a high-density portion unlike in the multi-drop scheme, an increase in print ink droplet amount and a decrease in print speed can be suppressed.
Furthermore, in this multi-density printing scheme, inks used to express an output image density corresponding to an input image density are determined using a multi-density ink distribution table (ink distribution table) shown in, e.g., FIG. 13. FIG. 13 shows an example of the contents of the multi-density ink distribution table using four types of multi-density inks. (Optical densities of A-D inks are 1:2:4:8, A and B inks are light inks and C and D inks are dark inks.) FIG. 14 is a flow chart showing the flow of an image process using the ink distribution table shown in FIG. 13.
The multi-density printing scheme rasterizes an input image in correspondence with individual inks using the multi-density ink distribution table based on the input image density. That is, in FIG. 14 an image is input (step S11), and undergoes a multi-valued process (step S12). Multi-density inks to be used are determined for the processed image using the multi-density ink distribution table (step S13).
After the multi-density inks used are determined, data is binarized by a binarization circuit (step S14) to generate drive signals for print element arrays a to d corresponding to inks A, B, C, and D (steps S15 to S18). In an image printed by such process, a low-density region such as an image highlight portion or the like is printed using light ink to reduce graininess of ink dots, and a high-density portion is printed using light and dark inks. In this way, the image quality can be improved.
In addition to grayscale expression in each pixel using a plurality of types of multi-density inks, grayscale expression based on dot size modulation by modulating the dot size of ink used in a print process is done. Note that the grayscale expression based on dot size modulation and that using multi-density inks are normally combined with the multi-drop scheme or pseudo halftoning rather than they are used alone.
As pseudo halftoning, dithering, error diffusion, average density preservation, and the like is known. Dithering binarizes each pixel data using threshold values for respective pixels determined by a dither matrix.
Error diffusion binarizes multi-valued image data of a pixel of interest (converts it into the darkest or lightest level), and distributes and adds the difference (error) between the converted binary level and the value before binarization to surrounding pixels, as described in, e.g., R. FLOYD and L. STEINBERG, xe2x80x9cAN ADAPTIVE ALGORITHM FOR SPATIAL GRAY SCALExe2x80x9d, SID 75 DIGEST, pp. 36-37.
Average density preservation obtains a threshold value on the basis of the already binarized binary data around the pixel of interest or values containing the binarization result of the pixel of interest to black or white, and binarizes image data of the pixel of interest using that threshold value, as described in, e.g., Japanese Patent Laid-Open No. 2-210962.
Furthermore, in addition to these methods, for example, when a transparent image of a medical X-ray film or the like is to be output, the resolution of vision with respect to density increases since it is a transparent image. As a result, even when the multi-density inks are used, the density differences for respective pixels are recognized, and an image may look coarse. In such case, the number of gray levels per pixel must be increased, and that requirement can be met by increasing the number of multi-density inks.
In the aforementioned ink-jet printing method, a multi-pass printing method is prevalently used. A multi-pass printing method using multi-density inks (described in, e.g., Japanese Patent Laid-Open No. 7-47698) forms an ink dot by controlling a plurality of ink droplets discharged from a plurality of print elements of a print head to land on substantially a single pixel, and expresses the gray level of that pixel by appropriately combining the number of ink droplets, and dark and light inks.
However, along with advance of the print technology, an input image to be processed has higher definition, and it is required to reproduce finer representation on a print medium. For this reason, both the input and output signals occupy huge capacity on a memory that stores these signals. In consideration of such situation, the conventional image printing method suffers the following problems.
(1) Ink discharged from a print head may deviate from a predetermined landing position due to nonuniformity of the shape of each ink ejection port of a print head and the surface state of a discharge surface. This phenomenon is normally called xe2x80x9ckinkxe2x80x9d. Especially, when a given gray level is printed over a predetermined area or more, and such image is formed to contain at least ink dots discharged from an identical print element, deviation of the landing position due to xe2x80x9ckinkxe2x80x9d is recognized as so-called xe2x80x9cstripesxe2x80x9d having periodicity, thus deteriorating image quality.
In order to eliminate the influences of xe2x80x9ckinkxe2x80x9d, the aforementioned multi-pass printing method is used. For example, the periodicity of xe2x80x9ckinkxe2x80x9d may be changed or distributed using a higher-order multi-pass printing method. However, printing by the higher-order multi-pass printing method requires a longer print time. Hence, xe2x80x9cstripesxe2x80x9d cannot be effectively reduced by a fewer number of passes.
(2) When a given print element does not eject any ink droplet over a predetermined area or more, an ink droplet the print element discharges first in that scan may form an ink dot different from that formed when the print element continuously discharges ink, due to a change in state of the ink ejection port of that print element. In this manner, a pixel having a gray level locally different from that to be printed may be generated in an image. This phenomenon tends to appear especially in low-density ink, and readily occurs in the multi-density ink printing method in this sense.
(3) The print head of an ink-jet printing apparatus cannot be permanently used, and may influence the print quality if it damages or stains. For this reason, the print head is exchanged. In this case, it is preferable to obtain equivalent printouts by print heads before and after the exchange. That is, print heads having different characteristics before and after the exchange may change image quality.
(4) When an input image is high-definition, high-resolution information, a gray level that a unit pixel can reproduce is limited. For this reason, upon printing that high-resolution image while its information size remains the same, pseudo halftoning is done. This process is time-consuming.
(5) The print speed can be improved by decimating pixels of an input image, but then the image quality drops.
The present invention has been made in consideration of the aforementioned problems, and has as its object to provide an ink-jet printing apparatus and method, which can print a high-definition input image having huge image information at high speed without deteriorating its image quality, and a computer readable memory.
According to the present invention, the foregoing object is attained by providing an ink-jet printing apparatus for printing a visible image on a print medium by discharging ink from a plurality of ink ejection print elements, comprising means for storing a first table indicating a correspondence between a plurality of inks and gray scale values of print pixels, means for storing a second table indicating combinations of density distribution patterns of print pixels and the ink ejection print elements in correspondence with the gray scale values, designation means for designating a region consisting of a predetermined number of neighboring pixels from pixels that form an input image, selection means for selecting the pixel density distribution pattern for the designated region, and control means for controlling ink ejection/non-ejection of ink from the plurality of ink ejection print elements by looking up the first and second tables in accordance with the pixel density distribution pattern and a gray scale value thereof.
According to the present invention, the foregoing object is attained by providing an ink-jet printing method for printing a visible image on a print medium by discharging ink from a plurality of ink ejection print elements, comprising a designation step of designating a region consisting of a predetermined number of neighboring pixels from pixels that form an input image, a selection step of selecting a pixel density distribution pattern for the designated region, and a control step of controlling ink ejection/non-ejection of ink from the plurality of ink ejection print elements by looking up a first table indicating a correspondence between a plurality of inks and gray scale values of print pixels and a second table indicating combinations of density distribution patterns of print pixels and the ink ejection print elements in correspondence with the gray scale values in accordance with the pixel density distribution pattern and a gray scale value thereof.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.